Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. Such EUV mirrors typically have a mirror substrate and a reflection layer stack—constructed from a multiplicity of layer packets—for reflecting the electromagnetic radiation incident on the optically effective surface. In the illumination device of a microlithographic projection exposure apparatus designed for operation in the EUV range, in particular the use of facet mirrors in the form of field facet mirrors and pupil facet mirrors as focusing components is known for example from DE 10 2008 009 600 A1. Such facet mirrors are constructed from a multiplicity of individual mirrors or mirror facets.
In practice there is often a need to measure the reflection properties of such mirrors or mirror arrangements with the highest possible accuracy and also—in particular in the case of mirror arrangements comprising comparatively small mirrors such as the aforementioned facet mirrors—to measure these properties with high spatial resolution. However, here the problem arises that an ever-increasing reduction in the respective measuring spots that is conceivable for this purpose has limits, to the extent that the reduction in the measuring spots is accompanied by an increase in the beam divergence or reduction in the intensity, which in turn has the effect that a relatively wide spectrum of different angles of incidence is achieved, which ultimately impairs the spectral resolution achieved in the measurement.
It is also known for the spatially resolved measurement of the reflection properties of a reflective optical element to arrange a detector, for example in the form of a CCD camera, in the far field of the reflective surface to be examined, which is illuminated over its full surface area. However, here the further problem arises that roughnesses of the reflective surface to be examined (unless for example it is well polished or the reflection coating is uniformly thick) lead to deformations of the wave fronts that are respectively reflected at this surface, which results in undesired interference effects at the detector. Since it is not easy to distinguish at the detector whether the measured variation in the intensity distribution is attributable to inhomogeneities in the reflectivity or unevennesses in the reflective surface examined, the interference effects mentioned lead to an impairment of the measuring accuracy.
As prior art, reference is made only by way of example to U.S. Pat. No. 2,759,106, U.S. Pat. No. 7,016,030 B2 and the publication U. D. Zeitner et al.: “Schwarzschild-Objective-Based EUV Micro Exposure Tool”, SPIE-Proceedings volume 6151, March 2006.